LTE-Advanced (Long-Term Evolution) of cellular radio communications is standardized in 3GPP (3rd Generation Partnership Project). A general diagram of LTE as it may be interfaced with other networks is shown in FIG. 1.
The exponential growth in the demand for radio or “wireless” data communications has put tremendous pressure on cellular network operators to improve the capacity of their communication networks. Relay nodes can improve the coverage and capacity of radio communications networks. A relay node may be positioned between a radio base station and a mobile radio user terminal so that transmissions between that base station, referred to herein as the relay's “donor” base station, and the user terminal are relayed by the relay node.
LTE systems, e.g., 3GPP LTE Rel-10, may support Type 1 relay nodes, which appear to the user terminal (referred to in LTE as a user equipment (UE)) as a separate base station distinct from the donor base station. A base station is referred to as an enhanced NodeB (eNB) and a donor base station as a DeNB in LTE. The service areas covered by a Type 1 relay nodes, each serving one or several relay cells, referred to as relay cells, also appear to a user terminal as separate cells distinct from the cells of the donor base station. The relay cells controlled by the relay nodes include their own Physical Cell ID (as defined in LTE Rel-8), and the relay node transmits a synchronization channel, a reference symbol, etc. for each served relay cell. In the context of single-cell operation, the user terminal receives scheduling information and Hybrid Automatic Repeat-reQuest (HARQ) feedback directly from the relay node and sends control information, such as service requests (SRs), channel quality indications (CQIs), and acknowledgements (ACKs) to the relay node. A Type 1 relay node is backward compatible and appears as a base station to LTE Release 8 user terminals. Thus, from the perspective of a user terminal, there is no difference being served by a base station or a Type 1 relay node.
Transmissions between the relay node and the donor base station are over a radio interface called the Un interface in LTE. The Un interface, sometimes also referred to as the backhaul link, provides backhaul transport for data transferred between the relay node and user terminals connected to that relay node, and the core network. The LTE Rel-10 standard specifies radio protocols for the backhaul link. Transmissions between user terminal and relay node are over a radio interface called the Uu interface in LTE, which is also referred to as the access link. The radio protocols for the access link are the same in LTE as for direct radio communication between the user terminal and a base station (e.g., donor base station) without a relay node being located in between.
The relay node comprises two main parts: a user terminal part for communicating with the donor base station over the Un interface and a base station part for communicating with user terminals over the Uu interface. The user terminal part operates much like a normal user terminal. Thus, normal user terminal access procedures and methods are employed on the Un interface to establish connections between the relay node and the donor base station. These access procedures are described in 3GPP TR36.806, “Evolved Universal Terrestrial Radio Access (E-UTRA); Relay Architecture for E-UTRA (LTE-Advanced) (Release 9), incorporated herein by reference.”
When a relay node attaches to the LTE network, it may optionally re-use the LTE user terminal (UE) “attach” procedure in order to establish Internet Protocol (IP) connectivity with the core network. Once the attach procedure is completed, the relay node contacts an Operations and Maintenance (O&M) system or other network node in the core network in order to become active as a base station.
The UE attach procedure in LTE is designed so that the eNB does not need to know unique UE identifiers, e.g., IMSI, IMEI, etc. Only the core network (the evolved packet core (EPC) including the mobility management entity (MME) in LTE) is typically aware of these globally unique UE identifiers. The eNB is aware of local identifiers of the UE-specific radio resource control (RRC) connection between the UE and eNB (identified by a cell radio network temporary identifier (CRNTI)) and the UE-specific S1 connection between the eNB and the MME (identified by MME UE S1AP ID and eNB UE S1AP ID). Furthermore, temporary identifiers are assigned (like the Globally Unique Temporary ID (GUTI) which identifies the MME and the temporary mobile subscriber identifier (TMSI) which identifies the UE within the MME) so as to avoid in many cases the need to transmit unique UE identifiers (e.g., IMSI, IMEI) over the radio interface and via the eNB.
In addition to the identities described above, each E-UTRAN cell broadcasts one public land mobile network (PLMN) identity, (or several in a PLMN-IdentityList), a unique 28-bit cell identity of the cell within the context of a PLMN, and a physical cell identity (PCI). The PCI is mapped to synchronization signals broadcast in the cell that the UE uses for cell search and cell identification. The number of available PCIs is limited per frequency layer, which means that the PCIs need to be reused. Ideally, however, the PCIs are perceived as locally unique per frequency layer so that a UE can identify handover candidate cells in measurement reports by their corresponding broadcasted PCIs. The combination of PLMN (the first PLMN in case of a list) and the 28-bit cell identity that uniquely identifies the cell within the scope of the PLMN is denoted the Evolved Cell Global Identifier (ECGI). This unique cell identifier ECGI can be used to look up the connectivity information of the candidate cell resulting in the functionality commonly referred to as Automatic Neighbor Relations (ANR).
One problem is how to determine and set the initial ECGIs of the relay node (RN) cells. A possible solution is to set the ECGIs of the relay node cells to a value that is independent of the donor base station's identifier, e.g., a DeNB ID, and instead use a dedicated eNB ID to the relay node. But a drawback with this solution is that the relationship that exists between the relay node and its DeNB cannot then be derived from the ECGIs of the relay node cells. For example, consider a situation where a first cell served by a first eNB is a neighbor cell to a second cell served by a second eNB, the second eNB is also a DeNB to a relay node, and there is no neighbor relation between the any of the relay node cells and the first cell. A UE served by the first cell detects one of the relay node cells and reports to the first cell the PCI and then the ECGI of that relay node cell. But the first eNB serving that first cell does not know and cannot readily determine that the RN is served by the second eNB (a neighbor of the first eNB) to which the first eNB already has established connectivity. In order to handle this situation, the MME needs to keep track of the relay node connectivity, which requires significant effort because the number of deployed relay nodes may outnumber the number of deployed eNBs by an order of magnitude. This results in inefficient relay node handling.
A similar problem exists when the core network (MME) wants to reach a specific relay node under a donor eNB. S1 messages are routed based on the eNB ID. So for dedicated relay node eNB IDs, the MME must keep track of all the relay node eNB IDs.
Another issue with using a fixed ECGI for each relay node cell is in dynamic situations where the radio conditions, mobility, etc. changes. In a changed situation, another eNB may be a more favorable eNB to serve the relay node than its current DeNB. As a result, the relay node may need to be relocated to a new DeNB that has a different eNB ID than the old DeNB. The MME routing information about the relocated RN thus needs to be updated at every RN relocation which requires extensive reconfiguration efforts.
What is needed is a better way to determine a relay cell identity that overcomes the problems and drawbacks identified above.